RFC: Package Necessary Files To Build out-of-tree modules

[Federico Vaga]

Hello,

I did this little patch to do some experiments around the idea of having the
kernel building system being able to produce a tarball containing all the
required files for building out-of-tree modules. The direct advantage would be
mainly for distributions' packagers who do not need anymore to specify the list
of required files - for all distributions.

This is just a draft, to see if the idea has some value for the community. For
sure the list of files is incomplete, and in other cases it adds more than
necessary. I see `rsync` is already in use in other files, so I'm not
introducing a new dependency, but of course it can be easily replaced with
something else. I copied from `scripts/package/buildtar` the parts about
creating the tarball.

so, ready for comments :)